Secondary winding combination for potential type instrument transformers



Jan" 13, 1970 R R D'ENTR ONT 3,489,975

SECONDARY WI G COMBINATI FOR POTENTIAL TYPE I TR NT TR FORMERS il Aug. 1967 United States Patent 3,489,975 SECONDARY WINDING COMBINATION FOR P0- TENTIAL TYPE INSTRUMENT TRANSFORMERS Franklin R. DEntremont, Dover, N.H., assignor to General Electric Company, a corporation of New York Filed Aug. 31, 1967, Ser. No. 664,792 Int. Cl. H01f 27/28 US. Cl. 336182 1 Claim ABSTRACT OF THE DISCLOSURE An instrument transformer of the potential type in which the secondary windings are split into two separate coils. The number of turns on each coil of the secondary winding are different from each other and different wire sizes are used for each coil. The number of turns in each coil of the secondary win-ding is preselected and the wire sizes in the coils are selected to obtain a desired secondary resistance according to the desired ratio correction factor of the potential transformer.

BACKGROUND OF INVENTION transformer, the true ratio of transformation does not equal the turn ratio. These ratio errors can be reduced for some conditions by using a turn ratio slightly different from the marked ratio. For example, in the case of potential transformers, the number of secondary winding turns may be slightly increased. The transformer is then said to be compensated for the particular condition. The particular compensation for a given transformer depends largely on the characteristics, such as frequency, power factor, burden impedance and primary voltage or current.

In general, the ratio errors of a potential transformer are caused by the voltage drops which are due to the cur rents which flow through the impedances of the transformer windings. When there is no load on the transformer an error is produced by the voltage drop due t the exciting current which flows through the primary winding. When a load is placed on the transformer an addi tional error is caused by the voltage drop produced by the burden current flowing through the impedance of both the primary and secondary windings. To obtain the most desirable accuracy of the transformer over the range from no load to maximum rated load the transformer should be designed to provide the highest possible voltage at no load, within the limits set by the A.S.A. Standard. With this high no load voltage, as the burden increases the secondary voltage will not drop below the standards at rated load or burden.

As will be understood, with low ratio transformers, a single primary turn will provide a substantial percentage change in voltage compared to industry standards. In order to meet required industry standards, it is considered necessary to be able to obtain the best possible ratio c mpensation under no load conditions.

From the above it will be apparent that it is desirable to provide means for controlling the ratio error or the ratio correction of potential transformers. It is well known to use pairs of secondary windings split about the primary ICC winding to compensate for errors of transformers. An example of this type of compensation for a potential transformer is shown in Patent No. 1,979,096. However, such methods generally depend on using partial turns on each secondary winding to obtain the desired correction of ratio errors. As is well known, the use of partial turns requires the winding leads to exit at different points on the winding.

It has recently been discovered that if parallel secondary windings are used in a potential transformer, that the ratio correction factor of such transformer can be selected and the secondary windings wound with wire of predetermined size to obtain a resistance which is dependent on the ratio correction factor.

It is therefore one object of this invention to provide a potential transformer having parallel secondary windings.

A further object of this invention is to provide a potential transformer having parallel secondary windings of predetermined resistance to obtain a desired ratio correction factor.

A still further object of this invention is to provide a potential transformer in which parallel secondary windings are provided with each such secondary winding having a resistance which depends on the desired overall secondary resistance and the ratio correction factor.

SUMMARY OF INVENTION In carrying out this invention in a preferred form, a potential transformer is provided having a primary winding and a pair of secondary windings connected in parallel circuit relation. The primary and secondary windings are electromagnetically linked with an electromagnetic core. The ratio correction factor and the secondary resistance of the transformer are preselected according to the accuracy requirements and the load characteristics of the transformer. The secondary windings are each provided with a different preselected number of turns and the size of the wire of each secondary winding is selected to pr vide a required resistance of each such winding according to the predetermined ratio correction factor and overall secondary resistance.

The invention which is sought to be protected will be particularly pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner in which its objects and advantages are obtained as well as other objects and advantages thereof, will be better understood by reference to the following detailed description of a preferred embodiment thereof, especially when considered in the light of the accompanying drawing.

BRIEF DESCRIPTION OF DRAWING The single figure of drawing is a schematic illustration of a potential transformer constructed in accordance with a preferred embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENT According to this invention, a potential transformer, such as illustrated in the drawing, may be constructed to obtain the most desirable turn ratio or ratio correction factor. As is well known, the turn ratio or the ratio correction factor is based on the exciting current required for the transformer and the impedence of the primary winding. The most desirable ratio correction factor for potential transformer will be in the range of .9965 to .9969.

Referring now to the drawing, a preferred construction according to this invention is shown schematically as a potential transformer 10 which includes a primary winding 12, the primary winding 12 is shown as being connected between a high voltage line 14 and ground 16. The winding 12 is electromagnetically linked with a core 18 as will be understood by those skilled in the art. A secondary winding is provided in the form of a pair of secondary coils 20 and 22 which are also designated as S and 8;, respectively. Secondary coils 20 and 22 are connected electrically in parallel and are also electromagnetically linked to the core 18 in the usual manner. Leads 24 and 26 from the parallel connected secondary winding are connected to a secondary burden or load 28 which may be, for example, a voltage meter.

According to this invention, it has been found that the most desirable ratio correction factor can be obtained by selecting the wire size of the wires used to wind the secondary coils 20 and 22 to obtain a constant ratio of resistance K. The constant K is determined by the desired ratio correction factor, the basic turns of the transformer and the number of turns on the secondary coil, that is different from the basic turns. The following symbols will be'used to indicate the described terms in the examples which follows:

of coils S and S Below are set forth examples which willillustrate the resistance of each of the parallel secondary coils S and S when a set ratio correction factor has been determined or preselected. In the examples, the basic turn N will be considered equal to 160 turns and R the combined resistance of the secondary coils S and S connected in par allel will be considered as 0.44 ohms.

Example 1 where where N =161 turns, N =160 turns, such that n =1 and n R.C.F. K R51 R5,

Example 2, Where N =161 such that n =1 and n =1 R.G.F. K R51 Rsz From a review of the preceding examples, it will be apparent that to obtain a desired ratio correction factor it is only'necessary to determine the desired resistance of eachof the secondary windings which are connected in parallel and then to select a Wire such as at the desired number of secondary turns on the coil will provide the desired resistance. For example, considering Example 1. In order to obtain aratio correction of 0.9967, the wire size for the secondary winding S or would be selected such that 161 turns would provide a resistance of approximately 0.830 ohm, while the wire size for secondary winding S or 22 would be selected so that 160 turns would provide a resistance of approximately 0.936 ohm. With reference to the conditions of Example 2, to obtain a ratio correction factor of 0.9966, the wire size for secondary winding S would be selected such that 161 turns would provide a resistance of approximately 0.569 ohm while the wire size for secondary winding S would be selected so that 159 turns would provide a resistance of approximately 1.938 ohms. From the above it will be apparent that by using the construction of this invention, it is possible to make potential transformers having desired ratio correction factors in accordance with the particular load characteristics of such potential transformers.

While there has been shown and described the present preferred embodiment with reefrence to specific examples of potential transformers, it will, of course, be understood by those skilled in the art that various changes may be made in constructional details without departing from the spirit and scope of the invention particularly as it is defined in the appended claim.

What is claimed as new and which it is desired to secure by Letter-s Patent of the United States is:

1. In a potential transformer having a predetermined ratio correction factor and including a core member, a

' primary winding electromagnetically linked with said core member and a pair of secondary windings electromagnetically linked with said core member and connected in parallel circuit relation, the improvement which comprises: each of said pair of secondary windings having a different 1 number of turns and each of said pair of secondary windings having a wire size selected to provide a resistance for each of said pair of secondary windings (R and R determined according to said predetermined ratio correction factor (R.C.F.), the number of turns of each of said secondary windings (N and N and the secondary resistance of the potential transformer (R according to the formula Rs =Rs( K and .R =R (1+K) where K is a constant ratio of resistance determined by the formula 'NS, RoF +N (1-RCF) W where N is the basic secondary turns of the potentialtransformer.

References Cited UNITED STATES PATENTS 1,865,430 7/1932 Borkent 336172 XR 1,896,993 2/1933 Arland 336-182 XR 2,089,860 1 8/1937 Rypinski 336l82 XR 2,283,711 5/1942 Welch 336l72 3,360,754 12/1967 Gerdiman 336183 XR LEWIS H. MYERS, Primary Examiner T. J. KOZMA, Assistant Examiner US. Cl. X.R. 336-183 

